The present invention relates generally to a lead frame for use in an integrated circuit package and more specifically to a method of producing a lead frame by molding an electrically conductive material such as powdered copper metal into a desired lead frame shape.
The process of molding a powdered metal currently exists and is used to produce a wide variety of items. To date this technique has been considered a process which is used in situations that require an exotic alloy or some other specialized application. However, the applicant has determined that this technique can be applied to produce cost effective and improved lead frames.
Lead frames for use in an integrated circuit package are typically produced in one of two ways, either being stamped or etched from a sheet of rolled copper. Although this process is widely used and is commercially successful, there are some inherent disadvantages and limitations when using lead frames manufactured by this process.
One of the disadvantages of the current method of producing lead frames results from the process of rolling the copper sheet that is used as the raw material. When metal is rolled it tends to develop a grain in the direction that the material is rolled. After the material has been stamped or etched into the desired lead frame shape, the typical process of manufacturing an integrated circuit package involves wire bonding various components to the lead frame. However, the grain of the lead frame material can have a negative effect on the bonding characteristics of the wire bonding process, depending on the direction of the grain relative to the direction of the bonding process. This problem becomes more significant as the pitch or density of the leads on the lead frame increases. As will be seen hereinafter, lead frames produced in accordance with the present invention do not have the above mentioned grain and therefore do not produce the above mentioned bonding problem.
Another disadvantage with the current method of stamping or etching lead frames is the amount of material wasted. Both of these processes involve removing material from a copper sheet to produce the desired shape. All of the cut away or etched material must be scrapped. Again, as will be seen hereinafter, the present invention significantly reduces the amount of material wasted during the production of a lead frame by using only the appropriate amount of powdered material to mold the desired shape.
FIGS. 1 and 2 illustrate art example of an integrated circuit package, generally designated by reference numeral 10, supporting a lead frame 12 produced using the current method of stamping or etching the lead frame from a sheet of copper. Lead frame 12 includes an array of leads 14 and one or more component attach pads indicated by 18 and 20. Still referring to FIGS. 1 and 2, another problem encountered with stamped or etched lead frames is providing a means for mechanically attaching the lead frame to the integrated circuit package. Typically this is accomplished by stamping or etching a small locking hole 16 in each of the leads 14. When the integrated circuit package is molded around lead frame 12, the molding material fills locking hole 16 forming a small pin through hole 16. This pin mechanically attaches the lead to the package.
However, in fine pitch applications where the width of lead 14 gets smaller as the lead density increases, the locking hole method of attaching the lead to the package becomes less practical. When the width of the lead is reduced, the diameter of the locking hole must be reduced. As the diameter of the locking hole approaches the thickness of the lead, it becomes difficult to stamp or etch a locking hole because of the physical limitations of the stamping and etching processes. The present invention eliminates the need for a locking hole and provides an improved method of mechanically connecting the lead to the package, especially in a fine pitch application.
Another limitation associated with a stamped or etched lead frame is again associated with fine pitch applications where the density of the leads causes the width of the leads to approach the same dimension as the thickness of the leads. Both the etching process and the stamping process have certain lead spacing limitations under these circumstances. The present invention eliminates these limitations and allows lead frames to be produced wherein the lead thickness can be greater than the lead width in cases where this is desired.
Another significant disadvantage of the current method of producing lead frames is that the lead frame is limited to one thickness. Because lead frames are typically stamped or etched from a single sheet of material, designers are limited in what can be designed into the lead frame. For instance, still referring to FIGS. 1 and 2, a designer may wish to use component attach pad 18 as a heat sink. In this situation, the designer would be restricted to a heat sink with a thickness equal to the thickness of the component attach pad which, in some cases, may not provide a large enough heat Sink. To compensate for this restriction, a discrete heat sink might be added to the lead frame during the manufacture of the integrated circuit package. Alternatively, a heat conducting portion or path 22 of lead frame 12 might be added to a component attach pad 20 to allow the lead frame based component attach pad 20 to be connected to an external heat sink. However, having the thickness of the heat conducting path 22 limited to the thickness of the lead frame and a desire to avoid the need for a discrete or external heat sink prevents these approaches from solving the problem in all instances. A lead frame produced in accordance with the present invention allows a lead frame to be produced wherein the thickness of the lead frame can be varied to create a heat sink or a heat conducting path that suits the requirements of the application.